I've never flown a Dash 8, but I have fair experience in DHC-6 and Caravan. On both those types, rapidly moving the propeller levers from coarse pitch to fine pitch can get you close to an overspeed, though 60% over seems very hard to imagine. The key is that power lever movement alone should not really result in significant overspeed, as the governor should govern. I speculate that overspeed risk is cause by mishandling the propeller levers.
What would happen if the propeller controls were moved toward feather in error, and then rapidly moved back to fine to correct things, while lots of power was applied? Could the governor change the blade angle quickly enough to prevent an overspeed?
Though I respect what Bombardier would instruct about the operation of the Dash 8, I would speculate that if a pilot selected power below flight idle in flight, that would initially result in a lower propeller speed, as there would not be much engine power commanded to turn the discing propellers. The discing propellers would have some of the blade in reverse pitch, thus large torque required to turn the prop.
Just me thinking, I have no knowledge of the accident....